Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR GENERATING ELECTRICITY AND CONCENTRATED
CARBON DIOXIDE
The present invention relates to a process for the
generation of electricity and the production of a
concentrated C02 stream using a molten carbonate fuel
cell (MCFC).
Molten carbonate fuel cells comprise a plurality of
fuel cell elements, each comprising an electrolyte layer
sandwiched between an anode. layer and a cathode layer.
The electrolyte layer is a porous layer soaked with
molten carbonate. Charge transfer through the electrolyte
layer from the cathode to the anode is done by carbonate
ions.
Fuel gas which is reformed in the anode chamber or
reformed fuel gas is fed to the anode chamber. If the
fuel gas is methane, the following reforming reaction
takes place:
CH4 + H2O -> CO + 3 H2 (1)
Carbon dioxide and oxygen are fed to the cathode chamber.
The cathode reaction in a molten carbonate fuel cell is:
4 C02 + 2 02 + 8e-4 4 CO32-; (2)
and the anode reactions are:
3 CO32- + 3 H2 - 3 H2O + 3 C02 + 6e- (3)
C032- + CO - 2 C02 + 2e- (4)
The overall reaction is:
CH4 + H2O + 2 02 3 H2O + C02 (5)
In a conventional operation of a MCFC, the anode
off-gas is recycled, typically after combustion of the
non-utilised hydrogen and carbon monoxide, to the cathode
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chamber to provide for the carbon dioxide needed at the
cathode layer. Air is fed to the cathode chamber to
provide for the oxygen needed. The exhaust gas of the
system, i.e. the cathode off-gas, comprises diluted
carbon dioxide, usually in a concentration of
about 3-5 % (v/v).
In order to minimise the amount of carbon dioxide
emitted to the atmosphere, it is advantageous to operate
a MCFC in such a way that the carbon dioxide produced is
obtained in a concentrated form. Carbon dioxide in a
highly concentrated form, typically above 80 % (v/v), can
be efficiently liquefied and subsequently used in
enhanced oil recovery or the recovery of coal bed
methane. Also for effective sequestration of carbon
dioxide, a concentrated carbon dioxide stream is needed.
Carbon dioxide concentrated to about 50 % (v/v), can
usefully be applied in the food and paper industry.
In EP 418 864 A, a process is described wherein
carbon dioxide containing combustion exhaust gas is fed
to the cathode chamber of a MCFC. A high-concentration
carbon dioxide gas is recovered from the anode off-gas.
In the process of EP 418 864 A, there is still a diluted
carbon-dioxide containing gas stream emitted to the
atmosphere, i.e. the cathode off- gas. Moreover, the
anode off-gas from which carbon dioxide is to be
recovered is diluted with nitrogen.
Therefore, there is a need in the art for processes
wherein the carbon dioxide in the exhaust stream is
present in a high concentration, preferably at a
concentration above 80 volume %.
It has now been found that a molten carbonate fuel
cell can be operated in such a way that a stream of
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highly concentrated carbon dioxide is produced as exhaust gas.
Accordingly, the present invention relates to a process for the generation of
electricity
and the production of a carbon dioxide stream, especially a concentrated
carbon
dioxide stream, using a molten carbonate fuel cell, the fuel cell comprising
an
electrolyte sandwiched between an anode and a cathode, an anode chamber and a
cathode chamber, wherein
- a fuel gas is fed to the anode chamber and a cathode inlet gas comprising
carbon
dioxide and molecular oxygen is fed to the cathode chamber,
- the anode and cathode reactions are allowed to take place to produce
electricity, an
anode off-gas and a cathode off-gas,
- the anode off-gas is at least partly fed to a catalytic afterburner wherein
it is oxidised
with an oxidant to obtain an oxidised anode off-gas,
- the remainder of the anode off-gas is recycled to the anode chamber,
characterised in that:
- the oxidant comprises at least one of:
- part of the cathode off-gas, and
- part of a molecular oxygen containing external oxidant stream, which
external oxidant stream comprises at most 20% (v/v) nitrogen;
- the oxidised anode off-gas is brought into heat-exchange contact with the
remainder
of the cathode off-gas and the remainder of the external oxidant stream to
obtain
cooled anode off-gas and a heated mixture of cathode off-gas and external
oxidant;
- the cathode off-gas is cooled before being brought in heat-exchange contact
with the
oxidised anode off-gas;
- the cooled anode off-gas and the heated mixture of cathode off-gas and
external
oxidant are fed to the cathode chamber as the cathode inlet gas;
- as soon as a set point in the carbon dioxide concentration at the cathode
chamber
outlet in a range of 5 to 40 % (v/v) is reached, part of the cooled anode off-
gas is
withdrawn from the process.
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In the process according to the invention, cathode off-gas which is diluted
with a
molecular oxygen containing external oxidant stream which comprises at most
20%
(v/v) of nitrogen, and oxidised anode off-gas are fed to the cathode chamber.
The
anode off-gas is catalytically oxidised with relatively pure oxygen, i. e.
cathode off-
gas and/or the external oxidant stream. Thus, the amount of nitrogen or other
inert
gases in the system is minimised, resulting in an oxidised anode off-gas
stream which
contains mainly carbon dioxide and water. From this stream, highly
concentrated
carbon dioxide can easily be withdrawn after separation of the water from it,
e. g. by
condensation.
The external oxidant stream, which is used as oxidant in the catalytic
afterburner
and/or as oxidant for the cathode reaction, contains preferably at most 10%
(v/v)
nitrogen or other inert gases. The external oxidant stream may contain carbon
dioxide.
The carbon dioxide content of the external oxidant stream is not critical. The
external
oxidant stream preferably contains at least 70% (v/v) molecular oxygen, more
preferably at least 80% (v/v). An external oxidant stream of substantially
pure oxygen
is most preferred. Such a stream may suitable be produced by techniques known
in
the art, for example by the removal of nitrogen from air by pressure swing
absorption
(PSA).
The amount of external molecular oxygen containing oxidant added to the
process
will be determined by the amount of oxygen needed to sustain the
electrochemical
DOCSMTL: 4420261\1
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reaction of the fuel cell and will thus depend on the
oxygen concentration in the external oxidant stream.
The fuel gas fed to the anode chamber is preferably a
gaseous hydrocarbon gas such as natural gas, methane,
biogas, or land-fill gas that is reformed in the anode
chamber. Reference to reforming is to the reaction of the
fuel with steam to form carbon monoxide and hydrogen,
such as given by equation (1) for methane. An advantage
of reforming in the anode chamber or internal reforming
is that the heat produced by the charge transfer in the
electrolyte layer can then be directly used for the
endothermic reforming reaction. Reforming in the anode
chamber is typically achieved by placing an additional
Ni-containing reforming catalyst in the anode chamber.
If the fuel gas contains substantial amounts of C2+
hydrocarbons, it is preferred that the fuel is pre-
reformed before entering the anode chamber. In that case,
a hydrocarbonaceous fuel is converted by steam reforming
into a carbon monoxide and hydrogen containing gas
upstream of the anode chamber. The carbon monoxide and
hydrogen containing gas is then fed to the anode chamber
as fuel gas. Reforming upstream of the anode chamber may
be performed inside or outside the fuel cell.
The steam needed for the reforming reaction may be
provided by an external source, but is preferably
provided by the anode off-gas. In that case, part of the
anode off-gas is fed to the reformer or to the anode
chamber if reforming takes place inside the anode
chamber. Preferably, 35 to 90 % (v/v) of the anode-off
gas is recycled to the reformer or the anode chamber,
more preferably 50 to 80 % (v/v).
The fuel gas preferably contains less than 20% (v/v)
of nitrogen or other inert gases, more preferably less
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than 10% (v/v). Most preferably, the fuel gas contains
substantially no nitrogen. The fuel gas may contain a
substantial amount of carbon dioxide, for example up to
70% (v/v).
5 The anode off-gas is at least partly fed to a
catalytic afterburner. If not all of the anode off-gas is
fed to the afterburner, the remainder is fed to the anode
chamber either directly or indirectly via a steam
reformer upstream and in fluid communication with the
anode chamber for the purpose of providing the steam
needed for the steam reforming reaction as explained
hereinabove.
In the catalytic afterburner, the unconverted carbon
monoxide and hydrogen in the anode off-gas are oxidised.
The amount of oxidant fed to the catalytic afterburner is
preferably the stoichiometric amount needed for oxidising
the hydrogen and carbon monoxide. Thus, an oxidised anode
off-gas containing carbon dioxide, steam and
substantially no oxygen is obtained. The oxidant used in
the catalytic afterburner might be part of the cathode
off-gas, part of the external oxidant stream comprising
at most 20% v/v of nitrogen or a combination of both.
The oxidised anode off-gas is brought into heat
exchange contact with the remainder of the cathode off-
gas and the remainder of the external oxidant stream to
obtain cooled anode off-gas and a heated mixture of
cathode off-gas and external oxidant, which are both fed
to the cathode chamber as the cathode inlet gas.
The heat exchange contact serves to bring the cathode
inlet gas to the appropriate cathode inlet temperature.
In the process according to the invention, the
cathode inlet gas will provide for cooling of the fuel
cell, i.e. of the stack of fuel cell elements each
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containing'an electrolyte layer sandwiched between an
anode layer and a cathode layer. Therefore, the
temperature of the cathode inlet gas will be lower than
the temperature of the cathode off-gas. In order to
maintain the cathode inlet gas at the appropriate cathode
inlet temperature, the cathode off-gas is cooled before
being brought in heat-exchange contact with the hot
oxidised anode off-gas. In order to achieve cooling of
the stack by the cathode inlet gas, it is preferred that
the cathode inlet gas flow is higher than the flow that
contains the stoichiometric amount of oxygen needed to
maintain the electrochemical reaction. Preferably, the
cathode flow is 3-6 times the stoichiometric flow.
Cooled anode off-gas and the heated mixture of
cathode off-gas and external oxidant are fed to the
cathode chamber until a set point in carbon dioxide
concentration at the cathode chamber outlet is reached.
As soon as the set point is reached, part of the cooled
anode off-gas is withdrawn from the process. Preferably,
the amount of withdrawn anode off-gas is such that the
amount of carbon in the withdrawn gas is equal to the
amount of carbon in the fuel gas fed to the anode
chamber. In this way, the carbon dioxide concentration at
the outlet of the cathode chamber is kept constant at the
set point. It will be appreciated that the set point in
carbon dioxide concentration at the cathode chamber
outlet should be chosen such that the efficiency and life
time requirements of the fuel cell are sufficiently met.
Preferably, the set point is in the range of
from 5 to 40 % (v/v), more preferably of
from 10 to 30 % (v/v).
The withdrawn anode off-gas comprises mainly carbon
dioxide and steam. If there was nitrogen present in the
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external oxidant stream or in the fuel gas fed to the
anode chamber, the withdrawn anode off-gas will also
comprise nitrogen. In the preferred case, wherein
substantially pure oxygen is used as the external oxidant
and nitrogen-free fuel gas is fed to the anode, the
withdrawn anode off-gas consists substantially of carbon
dioxide and steam.
.A concentrated carbon dioxide stream can be obtained
by further cooling the withdrawn anode off-gas to a
temperature at which the steam condenses. Water can thus
be easily separated from the withdrawn anode off-gas.
It will be appreciated that for start-up of the
process according to the invention, fuel gas and external
steam will be fed to the anode chamber and burned anode
off-gas and oxygen will be fed to the cathode chamber.
The invention will be illustrated by means of
schematic Figures 1 to 4.
Figure 1 shows a conventional process for operating a
molten carbonate fuel cell.
Figure 2 shows a process according to the invention
wherein a mixture of cathode off-gas and external oxidant
is used as oxidant for the catalytic afterburner.
Figure 3 shows a process according to the invention
wherein only cathode off-gas is used as oxidant for the
catalytic afterburner.
Figure 4 shows a process according to the invention
wherein only external oxidant is used as oxidant for the
catalytic afterburner.
In Figure 1 is shown part of a molten carbonate fuel
cell 1 comprising an element 2 of an electrolyte layer
sandwiched between an anode layer and a cathode layer.
Fuel gas is fed to anode chamber 5 via line 6. The anode
off-gas is discharged from anode chamber 5 via line 7.
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The main part of the anode off-gas is led via line 8 to
catalytic afterburner 9. Air is fed to catalytic after
burner 9 via line 10. The remaining carbon monoxide and
hydrogen in the anode off-gas is oxidised in catalytic
afterburner 9. The oxidised anode off-gas and air are fed
via lines 11 and 12, respectively, to cathode chamber 13.
Part of the anode off-gas is recycled to anode chamber 5
via line 14. Cathode off-gas is discharged from cathode
chamber 13 via line 15.
In Figure 2 is shown part of a molten carbonate fuel
cell 1 operated according to a process of the invention.
The cathode off-gas is led via line 16 to heat
exchanger 17, wherein it is cooled to ambient
temperature. Water is thus condensed from the cathode
off-gas and withdrawn via line 18. Substantially pure
oxygen from a PSA unit (not shown) is fed via line 19 to
the cooled cathode off-gas in line 20, thus obtaining a
mixture of cathode off-gas and oxygen. The amount of the
mixture to provide for the amount of oxygen needed for
combustion of the unconverted carbon monoxide and
hydrogen in the anode off-gas is fed via valve, 21 and
line 22 to catalytic afterburner 9. The remainder of the
mixture is led via line 23 to heat exchanger 24. Oxidised
anode off-gas is led via line 25 to heat exchanger 24.
In heat exchanger 24, the hot oxidised anode off-gas
heats the cold mixture of cathode off-gas and oxygen to
the appropriate cathode inlet temperature. Cooled anode
off-gas and heated mixture of cathode off-gas and oxygen
are fed into cathode chamber 13 via lines 26 and 27,
respectively. Part of the cooled anode off-gas is
withdrawn from the process via line 28.
In Figure 3 is shown part of a molten carbonate fuel
cell 1 operated according to a process of the invention.
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The process is similar to that shown in Figure 2, but now
a substantially pure oxygen stream is fed by line 29 to the cooled
cathode off-gas in line 23. Thus, the external oxidant
stream bypasses catalytic afterburner 9. The oxidant fed
to catalytic afterburner 9 via line 22 is cathode off-gas
only.
In Figure 4 is shown part of a molten carbonate fuel
cell 1 operated according to a process of the invention.
The process is similar to that shown in Figures 2 and 3,
but now all of the cooled cathode off-gas is led from
heat exchanger 17 to heat exchanger 24 via line 20. The
oxidant for catalytic afterburner 9 is substantially pure
oxygen which is fed to afterburner 9 via valve 30 and
line 31. The remainder of the substantially pure oxygen
stream is fed to line 20 via line 32.
The invention will be further illustrated by means of
the following non-limiting example.
EXAMPLE
In an on-stream process in a fuel cell system as
shown in Figure 2, 1.0 NL/s of fresh methane is supplied
via line 6 to anode chamber 5 and a stream of 19.7 NL/s
containing 64 % (v/v) 02, 29 % (v/v) C02 and 7 % (v/v)02
and having a temperature of 600 C is supplied to cathode
chamber 13 via lines 26 and 27. 25.6 NL/s anode off-gas
having a temperature of 675 C and containing 63 % (v/v)
C02, 27 % (v/v) H20, 4 % (v/v) H2 and 5 % (v/v) CO is
discharged from anode chamber 5 via line 7. 19.2 NL/s of
the anode off-gas is recycled to anode chamber 5 via
line 14, 6.4 NL/s of the anode off-gas is fed to
catalytic afterburner 9 via line S. The anode inlet
temperature is 600 C. A stream of 14.6 NL/s of cathode
off-gas having a temperature of 675 C and
containing 75 0 (v/v) 02, 15 % (v/v) C02 and 10 % (v/v)
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H2O is discharged from cathode chamber 13 via line 16 and
cooled to room temperature in heat exchange 17. 1.5 NL/s
of water is separated from the cathode off-gas via
line 18. To the remaining 13.1 NL/s of cooled cathode
5 off-gas, 2.0 NL/s of substantially pure oxygen is added
via line 19. Of the resulting mixture that
comprises 85 % (v/v) 02 and 15 % (v/v) C02, 0.4 NL/s is
supplied via line 22 to catalytic afterburner 9 as
oxidant and 14.6 NL/s is led to heat exchanger 24 via
10 line 23. In afterburner 9, the anode off-gas is oxidised
to an oxidised anode off-gas containing 69 % (v/v) C02
and 31 % (v/v) H2O. 1.5 NL/s of the oxidised anode off-
gas is withdrawn from the system via line 28. The
remainder of the oxidised anode off-gas and the heated
oxidant make up the cathode inlet stream of 19.7 NL/s.
Reference herein to NL/s is to litres at standard
temperature and pressure conditions (STP; 0 C and
1 atm.) per second.
